1. Field of the Invention
The present invention relates to a thermosetting powder coating composition, which is superior in fluidity and coating operability and can form a coating film having excellent smoothness, finishing appearance and scratch resistance, and a coated article obtained by using the same.
2. Description of Related Art
Powder coating compositions have widely been used as an environmentally friendly coating composition, which do not emit organic solvents into the air during coating, in the entire metallic coating field. Among these powder coating compositions, a thermosetting powder coating composition has been used for various purposes because of its excellent coating performances.
However, a conventional coating composition in the field of the coating compositions had problems in that a relatively large film thickness and a high curing temperature are required so as to obtain the same appearance as that in the case of using a solvent-based coating composition. In the field of industry that requires coatings, such as the automotive industry or can manufacturing industry, there has been a strong desire to develop a coating composition which forms a thin film having good surface appearance and maintains stable quality even when coating continuously, in view of cost reduction and weight reduction.
In the process for preparing a coating composition using a melt-kneading process, which is generally used as the process for preparing a conventional coating composition, each particle diameter of a coating composition is reduced to improve the smoothness of the thin film. However, since mechanical grinding process is used to reduce the particle diameter, the cost for preparation of the coating composition increases. There is also a problem in that it is very difficult to reconcile good coatability with good appearance of the coating film because the particle diameter and the particle configuration are not easily controlled.
To cope with this problem, a powder coating composition for imparting excellent finishing appearance and a coating process thereof are disclosed in U.S. Pat. No. 5,948,866. Since a coating film having excellent finishing appearance and scratch resistance can be obtained when using the powder coating composition, the publication discloses that it is enable to apply the powder coating composition for purposes such as automobile top clear coating, where the powder coating composition has hardly been used because the resulting thin film has poor finishing appearance, by making the best use of the feature.
Although the powder coating composition is certainly superior in the finishing appearance and scratch resistance as compared with a conventional powder coating composition, the powder coating composition is still inferior in fluidity, coating operability and storage stability because it was prepared by the melt-kneading process. There is further scope of improvement.
According to this melt-kneading process, for example, a powder coating composition is prepared by mixing raw materials such as resins, curing agents, plasticizers, stabilizers, fluidity modifiers, pigments and fillers using a high speed mixer such as a Henschel mixer, melt-kneading this mixture in a high shear mixer such as extruder, thereby completely dispersing the respective raw materials, cooling the melt-kneaded mixture, grinding it into powders, and classifying the powders.
Since the powder coating composition is prepared through these processes, the thermosetting resin and the respective components such as curing agent are not homogeneously mixed sufficiently. When small particles are formed, the particle size distribution of ground particles is likely to become broad, and the particles are of non-uniform shape. Therefore, the fluidity of the powder coating composition is lowered and an adverse influence is likely to be exerted on the coating operability and storage stability of the powder coating composition. In particular, the appearance of the coating film is likely to become poor in the formation of the thin film.
An object to be achieved by the present invention is to provide a powder coating composition, which is superior in fluidity and coating operability and forms a coating film having excellent smoothness, finishing appearance and scratch resistance even when the resulting film has a small thickness such as 50 xcexcm or less, and a coated article thereof.
The present inventors have intensively researched to achieve the object described above and have found that a coating film formed by using a thermosetting powder coating composition comprising, as a coating film forming component, particles containing specific components, wherein an average particle diameter of entire particles is within a range from 5 to 50 xcexcm, an average circularity of the particles is 0.90 or more, and a circularity of particles having a maximum number frequency in circularity distribution is within a range from 0.98 to 1.00, is noticeably superior in smoothness when formed into a thin film, and is also superior in fluidity and electrostatic coatability. Thus, the present invention has been completed.
The present invention provides a thermosetting powder coating composition comprising, as a coating film forming component, particles containing an epoxy group-containing vinyl copolymer (A) having a glass transition temperature of 40xc2x0 C. or higher, an epoxy group-containing vinyl copolymer (B) having a glass transition temperature of 0xc2x0 C. or lower and a polycarboxylic acid (C) as an essential component, wherein an average particle diameter of entire particles is within a range from 5 to 50 xcexcm, an average circularity of the particles is 0.90 or more, and a circularity of particles having a maximum number frequency in circularity distribution is within a range from 0.98 to 1.00, and a coated article comprising an article to be coated and a coating film formed on the article to be coated by thermosetting the powder coating composition.
The present invention also provides a process for preparing a thermosetting powder coating composition having the feature described above by spray-drying a raw powder coating composition solution comprising an epoxy group-containing vinyl copolymer (A) having a glass transition temperature of 40xc2x0 C. or higher, an epoxy group-containing vinyl copolymer (B) having a glass transition temperature of 0xc2x0 C. or lower and a polycarboxylic acid (C) as an essential component.
The coating composition and the coated article according to the constitution of the present invention are superior in smoothness and scratch resistance in the form of a coating film, and the coating composition is superior in fluidity and coating operability.